Binghamton University. CS-140 Fall Subtyping!

Transcription

1 Subtyping! 1

2 Sub-types (Not emphasized in the text) 2

3 Comparing Shape w/ Rectangle ll Shape Rectangle ll width height Rectangle is a subtype of Shape (the set of rectangles is a subset of the set of shapes) 3

4 Sub-type Concept Since the sub-type contains the super-type Objects of the sub-type have a dual nature sub-type or super-type An object of a sub-type class can be placed anywhere an object of the super-type can occur. For more technical details, see Subtyping in Wikipedia Where can the super-class type occur 4

5 Sub-type Arguments If Duck extends Bird { Assume the Vet class has a method defined as: public void workon(bird bird) {. Then, the following is legal Vet drx = new Vet(); Duck quack = new Duck(); drx.workon(quack); A subtype sub-type argument can be passed as an argument where the super-type supertype is is expected 5

6 Sub-type Return Values If Duck extends Bird { Then, Vet can have a method defined as: public Bird getpatient( ) { Duck quack = new Duck(); return quack; A subtype sub-type argument can be returned can be passed where the as an super-type argument is where the expected supertype is expected 6

7 Example Sub-type return public class Customer { private SavingsAccount savings; private CheckingAccount checking; Account largerbalance() { if(checking == null) { return savings; // can return null if(savings!= null && checking.getbalance()< savings.getbalance()) { return savings; return checking; 7

8 Assignment to a sub-type If Duck extends Bird { Then, you may code:. Bird bird; Duck quack = new Duck(); bird = quack; // Not allowed: quack=bird A sub-type may be assigned where the super-type is expected 8

9 Example Sub-type assignment public class Customer { private SavingsAccount savings; private CheckingAccount checking; Account largerbalance { Account larger=checking; if(checking == null) { larger=savings; // can be null else if(savings!= null && checking.getbalance()< savings.getbalance()) { larger=savings; return larger; 9

10 Overriding sub-type (new in Java 7) public class Bird { public Bird son(bird a) { return new Bird(); public class Duck extends Bird public Duck son(bird a) { return new Duck(); Duck.gen overrides Bird.gen! 10

11 Using Overridden method public class Flock { public static void main(string[] args ) { Bird bird = new Bird(); Duck quack = new Duck(); Bird bird2 = bird.son(quack); bird=quack; bird2 = bird.son(bird); Can pass sub-type where super-type is expected. Can assign sub-type where super-type is expected Invokes override Duck.son method! 11

12 Enumerated Types in Java public enum State { ON, OFF ; Creates a new data type called State So I can declare variables of this type, e.g. State halllight; Creates new literals for each enumerated value So I can assign values, e.g. halllight=state.on; So I can compare values, e.g. if (halllight==state.off) { Variables of this type can only have enumerated values 12

13 Making Sub-types Sub-types can occur in Java three different ways 1. Through sub-classing (as we have seen) 2. Through enum constants public enum State { ON, OFF ; State.ON & State.OFF, are both sub-types of the super-type State 3. Through implementation of an interface The interface is the super-type The class that implements the interface is the sub-type 13

14 Interfaces as Types Interface is a super-type of the class that implements the interface Enables Java to use interfaces in places we normally expect types! For instance, as a parameter: void addlist(list<int> x) { x is some list of integers x is an instance of any class which supports the List<int> interface! That means we can use x.size() or int element=x.get(i) Replace complicated class hierarchy with simpler interface based collections A class may have multiple interfaces, but only one superclass Classes may implement interfaces without inheritance! 14

15 Overthinking Hierarchy Can we have too much of a good thing? Design Patterns: Elements of Reusable Object Oriented Software (Erich Gamma, Richard Helm, Ralph Johnson) The added complexity of a deep inheritance hierarchy often outweighs the advantages! 15

16 Example: Exceptions Chapter 11, Figure 2 The Java 8 library currently defines 515 exception classes We care about two types Checked Exceptions (checked at compile time) Unchecked Expressions (not checked at compile time) But checked/unchecked not reflected in class hierarchy! java.lang.throwable (implements java.io.serializable)java.lang.error java.lang.annotation.annotationformaterror java.lang.assertionerror java.awt.awterror java.nio.charset.codermalfunctionerror javax.xml.parsers.factoryconfigurationerror javax.xml.stream.factoryconfigurationerror java.io.ioerror java.lang.linkageerror java.lang.bootstrapmethoderror java.lang.classcircularityerror java.lang.classformaterror java.lang.reflect.genericsignatureformaterror java.lang.unsupportedclassversionerror java.lang.exceptionininitializererror java.lang.incompatibleclasschangeerror java.lang.abstractmethoderror java.lang.illegalaccesserror java.lang.instantiationerror java.lang.nosuchfielderror java.lang.nosuchmethoderror 16

17 Instead of 17

18 Why not Implement with Interfaces? 18

19 More Polymorphism Overriding methods is a key feature of OO programming Subtypes often override one or more methods Suppose Bird has children Duck, Wren, and Robin; each with their own nest method Bird[] arr = { new Duck(), new Wren(); new Robin(); for (Bird b : arr) { b.nest(); Invokes 3 different nest( ) methods! 19